HashCash isn't a new idea, but it's being talked about again, which is a shame really because I haven't come across a single application where hashcash would work well. Adam Back lists a few at the end of the aforelinked paper, including flood limiting in Freenet. Ignoring the practical problems of integrating hashcash, the major problem is that it scales linearly. If I want to do 1 action, I pay x. If I want to do 5 actions I only pay 5x. There is no way to tell different requesting parties apart, so this is fundamental.

Remember that computers from 5 years ago are going to be about 10 times slower than today's, and you hardly want to cut them off. So you either set the cost far too high, or spammers aren't going to notice it because buying a cheap cluster to calculate hashes isn't really going to bother them. (or even just write a virus/worm to make all the poor Windows users do it for you).

And even in systems where he suggests that hashcash only kick in in a DoS situation (e.g. connection flooding) it doesn't provide "more graceful service" degradation as he claims. It simply moves the bottleneck from the CPU/network to the client, and the fastest client gets served first. (Which would be ok if all the attackers were much slower, but they aren't).

An interesting development would be a computer generatable (my spell checker doesn't like that, but I think it's ok) challenge that only humans could solve. Possibly rendering some text and then messing it up would require a human to solve. That might still be impractical, and spammers could simply hire a sweatshop to solve them all day, but it would be interesting.

That lawyer

Oh, and on the spamming front; that lawyer who got blacklisted wrote back:

When it comes to mail administration, it appears I was
several years behind the curve. Since my mail server
software, circa 1996, had been purring along quietly
without problems since it was new, I had never upgraded
it to a version capable of a higher degree of authentication.
I'm also old enough to remember when an "open relay" was
a relay intentionally left open for anyone to use, not
one merely susceptible to misuse. Thanks to all of the
readers who wrote to bring me into the new millennium.
Both my software and my definition are now upgraded.

At the same time, I labelled the blackhole list operators
"vigilantes" for good reason. It was always my understanding
that if you lie about your identity to gain access to
something that would be closed to you if you told the truth,
you've done something wrong. That's true whether you intend
to send spam or prevent it. As vile as spam is, the ends
don't justify the means. Regardless of whether my mail
server used to be "open" or not, I stand by the legal
analysis that placed fault on the blackhole operators who
forged their identity.